Tishnaa – means thirsty, in poetic context it translates as insatiable or eagerly
Tishnagii is another form of this lafz which means desire, or longing. Here is our post on tishnagii – https://urduwallahs.wordpress.com/2012/07/24/tishnagii-2/
Here is a couplet from Roshan Nadan’s ghazal ‘Baad muddat unhe dekh kar yun laga’ with the lafz ‘tishnaa’, wonderfully sung by Jagjit & Chitra Singh.
Tishnaa Nazarein Mili Shokh Nazaron Se Jab
Mai Barasne Lagi Jaam Bharne Lagaa
Saaqiyaa Aaj Teri Zaroorat Nahin
Bin Piye Bin Pilaaye Khumaar Aa Gayaa
When my eyes filled with longing and desire met your attractive daring look
Wine started flowing, Goblets began to get filled
O bartender, I don’t need you to pour a drink today
Even without drinking, I feel intoxicated
Here is a link to the ghazal:
Tishnagi is also a Pakistani TV drama that plays on Express Entertainment in Pakistan. While we have not seen the show, here is the synopsis and a show promo for anyone interested:
Tishnagi is a story of unrealized dreams, unfilled promises, and unraveled lives. It is the story of Manahil (Sarwat Gilani), a young girl whose rose-colored dreams are rudely shattered when life throws her a curveball, placing her in the middle of a never-ending nightmare. She, who wants to celebrate every moment of her married life, gets the biggest shock of her life when, after all that she has gone through, her beloved husband hands her a divorce, because he deems it in her best interest. It is also the story of Nabeel (Faysal Qureshi), an ambitious young man, whose life is abruptly cut short when he is afflicted with MS and sidelined. Once a robust man, he is reduced to watching helplessly as his previously pampered and delicate wife runs from pillar to post trying to make ends meet, so that she can afford to pay for housing and healthcare. This is also the story of Faraz (Aijazz Aslam) and Andaleeb (Mahnoor Baloch), an odd couple if ever there was one. Faraz, ostensibly a man looking for love, and Andaleeb, a narcissist, have a difficult marriage, but as the narrative unfolds it is quite evident that both are equally psychotic and require serious therapy – though not by the quack shown here. (Source: http://www.dramapakistani.net/tishnagi-an-overview/)
Promo:
urduwallahs 
Is it derived from the sanskrit word Trishna?
If it is derived from the Sanskrit word (which has the same meaning), that would be so interesting. The general perception of Urdu is that emotive or poetic lafz in the language are generally derived from Persian or Arabic, with everyday verbs or common words coming from other sources. Would be interesting to know of lafz used in shairi that derive from Sanskrit, or Turkish, or other Central Asian sources. There must be some. Could there even be lafz that have Dravidian roots?